1. Field of the Invention
This invention relates to an automatic transmission control system for a vehicle.
2. Description of the Prior Art
As is well known, the automatic transmission generally upshifts and downshifts according to a shift pattern map which is stored in a micro computer as a function of the engine load and the vehicle speed. That is, the shift pattern map has a plurality of upshift lines and a plurality of downshift lines, and when the operating condition of the vehicle as defined by the engine load and the vehicle speed moves across one of the shift lines, the automatic transmission shifts into the gear speed determined by the shift line the operating condition crossed.
When the engine is cold and the viscosity of the transmission oil is high, the time required for the transmission to actually begin shifting after the operating condition of the vehicle moves across the shift line is elongated. Accordingly, especially in the kick-down range where the throttle valve is substantially wide opened, the engine speed can excessively increase and sometimes can exceed an upper limit engine speed even when the transmission correctly upshifts according to an upshift line.
Thus, it has been proposed, as disclosed in Japanese Unexamined Patent Publication No. 63(1988)-30640, to change: the upshifting point (the engine speed above which the transmission is to upshift) when the maximum engine speed during the upshift is not in a predetermined engine speed range. However, the system disclosed in the Japanese patent publication is disadvantageous in that it takes a long time for the automatic transmission to be brought into a state in which the engine speed can be prevented from exceeding an upper limit during upshift in the kick-down range when the engine is started while the transmission oil is cold since the upshifting engine speed is changed so that the maximum engine speed during the upshift does not exceed the upper engine speed limit starting from the engine speed at which the transmission is to upshift according to the upshift lines which has been set for the normal transmission oil temperature.